1. Field of the Invention
The present invention relates to polishing of CVD diamond film. More particularly, the present invention relates to methods of polishing CVD diamond film by oxygen and a fluorinated gas in both low and high power density plasmas.
2. The Prior Art
Various methods for polishing the diamond surface of an object are well known in the art. For many of the applications in which diamond coated surfaces are employed, the fine polishing of the diamond surface is not only desirable, but it can also be critical. For example, a rough diamond surface on diamond coated tools, such as cutting inserts and round tools, limits the utility of these tools.
The fine polishing of the diamond surface can also be very difficult because the surface is non-planar or is in a complex shape. For example, diamond coated cutting inserts and diamond coated round tools are often non-planar, and the diamond domes on the front of aircraft are in a complex shape. The conventional polishing methods used to polish non-planar and complex shapes are expensive and difficult to implement. Conventionally, there are several methods which are used to polish diamond surfaces.
Classic methods use iron wheels and molten metal. In one variation of these methods, the diamond surface is mechanically polished using an iron surface and diamond dust. In another variation of these methods, a heated iron surface and hydrogen are used to create a chemical mechanical polishing action. Both of these methods are limited, however, because the shapes that can be polished are not very complex. These methods also have the undesirable side effect of easily contaminating the diamond surface.
High temperature anneals are also known as a method of polishing diamond. In these high temperature annuals, carbon is diffused from the diamond surface at a high temperature (900.degree. C.) and captured by metal such as Fe or Mn or rare earth metals such as Ce or La.
Electro-chemical methods of polish diamond have also been employed. In one of these methods, the diamond surface is placed in contact with a stabilized ceramic oxide supersonic conductor such as Y.sub.2 O.sub.3 :Z.sub.V O.sub.2 while placing a potential across the diamond/ceramic surface. The resulting electrochemical action polishes the diamond film surface.
High powered lasers have also been used to planarize and trim diamond surfaces.
None of these known methods are very suitable for polishing diamond surfaces that need to be highly polished and/or have complex, non-planar surfaces. In fact, several of these are very time consuming and expensive methods especially where the materials or surfaces must be replaced to continue polishing. Further, several of these methods are not susceptible to batch processing, because when either at least the size, shape and thickness of the object being polished is changed the polishing method must also be adjusted. Finally, several of this methods result in the contamination of the diamond surface being polished.
Given that the above methods were considered less than satisfactory, attempts were made to polish a diamond film surface by various etching methods, since it was known in the art that diamond can be etched by oxygen in a heated non-plasma environment. Unfortunately, when experiments were conducted to observe whether oxygen etching in a heated environment would be a suitable method for polishing diamond, it was found that this method led to an increase in the surface roughness of the diamond film. Apparently, the heated molecular oxygen attacked the diamond film preferentially at the grain boundaries between the diamond crystals, and led to the formation of needle-like structures (as shown in FIG. 1), rather than the desired smoothening of the diamond film surface.
In an attempt to control the oxygen etch and thereby polish the diamond film, experiments were done with a low power, low density oxygen plasma. As is well known in the art, the reactive ion etching of a substrate occurs due to two different actions. The first action is an anisotropic or directional etch, typically occurring in the vertical direction, and caused by bombardment of ions onto the surface of the diamond substrate. The bombardment is caused either by an induced or applied bias at the surface of the diamond substrate. The second action is an isotropic or non-directional etch, caused by reaction of the reactive chemical species in the plasma with the molecules on the substrate surface.
In the experiments, an attempt was made to control the isotropic and anisotropic etch conditions generated by the plasma at low temperatures to preferentially etch the high points of a diamond film by the known reactive species, oxygen. Unfortunately, though the process conditions were adjusted to control the anisotropic and isotropic etching, the oxygen species which are at least known to etch diamond in a non-plasma environment, showed very little evidence of etching the diamond film.
Clearly, given the inadequacy of the known methods of diamond film surface polishing, and the importance of diamond film surface polishing, there exists a need in the art for additional methods of diamond film polishing.